Conventional optical imaging systems for image reading devices incorporated in multifunctional peripherals and the like using an electro-photographic process include a reduction optical system that images an image while reducing it and a unity magnification optical system that images an image at unity magnification without reducing it.
In a reduction optical system, a reduced image is imaged on an image sensor whose size is smaller than the size of a document (for example, one-fifth to one-ninth the size of a document) by use of a plurality of flat mirrors and optical lenses, and then the image is read. In the reduction optical system, as an image sensor, a charge-coupled device, which is called a CCD (charge-coupled device) sensor, is used. The reduction optical system has the advantage of a deep depth of field. Here, the depth of field is a range in which, even when a subject (here a document) is deviated from the in-focus position along the optical axis, the subject appears to be in focus. This means that, with a deep depth of field, even when the document is deviated from a prescribed position, it is possible to obtain a satisfactory image.
On the other hand, the reduction optical system has the disadvantage of a very large optical path length (a distance over which light travels from the subject to the sensor) of 200 to 500 mm. In image reading devices, for the purpose of securing the optical path length in a limited space in a carriage, a plurality of flat mirrors are used to change the direction in which light travels. This undesirably leads to an increased number of components and thus to increased cost. In a case where a lens is used in the optical system, chromatic aberration occurs due to refractive index varying with wavelength. To correct the chromatic aberration, a plurality of lenses are required. Using a plurality of lenses in this way also results in increased cost.
In a unity magnification optical system, as disclosed in Patent Document 1, with a plurality of erect-image unity-magnification rod lenses arranged in an array, an image is imaged on an image sensor whose size is equal to the size of a document, and the image is read. In the unity magnification optical system, as an image sensor, a photoelectric conversion device, which is called CMOS (complementary MOS) sensor, is used. The unity magnification optical system has the advantage of, compared with the reduction optical system, a smaller optical path length of 10 to 20 mm and a smaller size. Imaging is performed by use of rod lenses alone and thus no mirrors as are needed in the reduction optical system are required. It is thus possible to reduce the thickness of a scanner unit incorporating a unity magnification optical system sensor, and thereby achieve a simple structure and hence reduced cost. On the other hand, the unity magnification optical system has a very small depth of field; thus, when a document is deviated from a prescribed position along the optical axis, differences in magnification among the individual lenses causes image blur that appears as greatly softened focus. This results in the disadvantage of inability to uniformly read a book document or a document with an uneven surface.
In recent years, other than the reduction optical system and the unity magnification optical system described above, as disclosed in Patent Document 2, a method has been proposed in which an image is read by use of a reflection mirror array in an imaging optical system. In this method, with a plurality of reflection mirrors arranged in an array, a document read in different reading regions corresponding to the reflection mirrors is imaged into a reduced inverted image on a sensor. Unlike the unity magnification optical system using a rod-lends array, one region is read and imaged with one optical system. By adopting a telecentric optical system as the imaging system, when a document is read on a region-to-region basis, no image blur occurs due to overlap among images at different magnifications; it is thus possible to suppress softened focus and achieve a multi-mirror reading method.
Moreover, in this method, the optical system uses mirrors alone, and thus unlike in a case where the optical system uses a lens, no chromatic aberration occurs. Accordingly, there is no need for correcting chromatic aberration, and thus the number of elements constituting the optical system can be reduced.